Inflammatory cytokines e.g. IFN-γ, serve as initiating stimuli for mesenchymal stem cell (MSC) immunosuppresive activity in vivo. Other inflammatory cytokines, such as TNF alpha, the molecule hemoxygenase I, and TLR ligation of MSC, may also provide such a response. Activated MSC's promote tissue regeneration in conditions such as aging, where regeneration is impaired. Wound healing in aged mammals is enhanced by restoring tensile strength to the levels of younger mammals. Activated MSCs were useful in treating wounds in diabetic primates.
Wound Healing
1. General
With increasing numbers of aging patients undergoing surgery, post-surgical wound care presents obstacles of delayed healing, increased wound infection, and a greater incidence of dehiscence. In the absence of surgery, chronic wounds observed in patients 60 years old or older have increased recurrence, duration, number of wounds and frequency of infection when compared to younger patients. Taken together, these findings indicate an emerging and growing problem in wound care.
The desirable effects of mesenchymal stem cells administered in wounds can be, in part, attributed to their ability to reduce inflammation and enhance regeneration through the recruitment of alternatively activated macrophages, and endothelial cells, and enhanced vascularity. Interestingly, patients of advanced age appear to have diminished capacities in macrophage function with altered production of cytokines, growth factors, expression of TLR, and signaling via the ERK-MAPK pathway. Because macrophages play a significant role in the initial phases of the inflammatory wound response and the subsequent growth phase response by secreting angiogenic growth factors, enhancing macrophage function could facilitate resolution of the impaired wound healing responses observed in the aged. Because wounds of individuals advanced in age suffer from the ability to adequately and thoroughly heal, more potent interferon gamma activated MSC may facilitate wound healing, restoring the tensile strength of the wounds of aged mammals to strengths more commonly observed in young mammals. Bone marrow derived mesenchymal stem cells, activated by interferon gamma, promoted wound tensile strength in wounds of aged mice and monkeys. This effect was due to the subsequent participation of host macrophages.
2. In Diabetic Patients
Diabetes is one of the most frequent causes of compromised wound healing. As a global health concern, the number of diabetic patients worldwide is projected to increase from 197 million reported in 2003 to 366 million by the year 2030. In the United States, the estimated cost of diabetes in 2007 was 174 billion dollars, 58 billion of which was spent on diabetes-related complications.
Chronic wounds in diabetic patients fail to heal due to diabetic macro- and microvasculopathy, macrophage dysfunction and reduced ability of the release and the migration of BM derived and tissue residing pluripotent adult stem cells that can conduct the coordination of the local healing process. Moreover, the low microvascular perfusion inhibits appropriate epidermal and endothelial cell migration of angiogenesis and reepithelisation.
Effective strategies like vascular bypasses and/or epidermal replacement (Apligraf or skin allografts) increase wound healing and closure rates. Even though they also secrete angiogenesis promoting cytokines (vascular endothelial growth factor (VEGF), epidermal growth factors (EGF), platelet-derived growth factor (PDGF), transforming growth factors (TGFs), interleukin (IL)-1α (which is chemotactic for keratinocytes and enhances collagen synthesis), and tumor necrosis factor-α (which enhances fibroblast transformation)) they still do not repair the complex microvascular dysfunction and altered macrophage functions.
New clinical strategies such as the dermal matrix scaffolds, stem cell therapies and local growth hormone replacements are aiming to restore sufficient blood supply and cytokine microenvironment in early inflammatory phase of the healing process to define a more regenerative pattern. Bioengineered matrix scaffolds appears to offer a more complete therapy to reach functionally and anatomically complete healing.
Integra, an FDA approved acellular dermal skin substitute build by extracellular matrix (collagen and GAG), has been reported as successful in the treatment of wide tissue defects or nonhealing wounds like venous leg ulcers.
Locally or systematically administered MSCs have demonstrated improvement in healing, angiogenesis, and collagen content in wounds through producing paracrine factors to recruit macrophages and endothelial cells for tissue regeneration.
MSCs in response to inflammatory environment and signaling (IFN-gamma, TNF-alpha, IL-1beta) increase their effect to inhibit T-cell 1, NK-cell B-cell activation through enhanced production of indoleamine 2.3-dioxygenase, Cox-2, and PGE-2
Graft V. Host Disease
Recent evidence suggests MSC interactions with macrophages reduce pro-inflammatory activity to an anti-inflammatory, pro-regenerative enterprise with increased capabilities to phagocytose apoptotic cells. Such MSC-mediated effects could be due to both cell-cell contact as well as release of paracrine factors. Production of growth factors and cytokines by MSC can be enhanced when MSC are exposed to inflammatory signaling from interferon gamma or tumor necrosis factor. Increased MSC potency, a consequence of ex vivo exposure to interferon gamma, has been demonstrated to powerfully prevent graft versus host disease when comparably, non-interferon gamma activated MSC had no effect.
Graft versus host disease, mediated by donor T cells, is a significant source of morbidity and mortality following allogeneic stem cell transplantation. Mesenchymal stem cells (MSC) can successfully treat ongoing graft versus host disease, presumably due to their ability to suppress donor T cell proliferation. However, little is known about the mechanisms which MSC exert in vivo to prevent graft versus host disease which limits predictions of conditions for optimal effects.
Allogeneic hematopoietic stem cell transplants have the potential to play a significant curative role in the treatment of malignant and non-malignant hematopoietic disorders, autoimmune diseases, immunological deficiencies, and in the induction of transplantation tolerance. Unfortunately, widespread application of this therapeutic modality is limited due to the morbidity and mortality of graft versus host disease (GVHD), which affects 50% of stem cell transplant recipients. Although grafts highly matched to the recipient, young donors, donor/recipient sex match, and post-transplant immunosuppression are strategies used to reduce the risk of GVHD, thus far, the greatest preventative measure has been intentional underutilization of stem cell transplantation. Theoretically, strategies aimed in preventing GVHD would target early initiating factors either during the inflammatory milieu created in the wake of tissue damage from conditioning regimens, or during T cell antigen recognition and proliferation. After the efferent effector phase occurs, donor T cell mediated destruction of host tissues occurs and preventive strategies are replaced with treatment regimens.
Mesenchymal stem cells (MSC) have been used in the efferent phase of GVHD to treat ongoing, acute, steroid resistant GVHD. In contrast, when given at the time of bone marrow transplant, for the prevention of GVHD, the incidence of grade III/IV GVHD was not significantly improved. MSC reliably suppressed large scale T cell proliferation in response to polyclonal stimulation in vitro. In contrast, with allogeneic mixed lymphocyte cultures of variable stimulation, MSC suppression is also variable; MSC do not completely abrogate lymphocyte proliferative responses between all donor and recipient pairs. In addition, MSC do not suppress the modest T cell proliferative response to recall antigens.
Murine experimental models used to dissect the mechanism of MSC effects in the course of GVHD have yielded mixed results, with some studies showing MSC efficacy and others finding no effect. Several factors are likely to contribute to the variable results. MSC tissue source, (i.e., bone marrow, cord blood, adipose tissue), method of isolation to remove myeloid precursors (several weeks vs. rapid immunodepletion) and timing of MSC administration are potential variables which could explain these differences. Notably, such variation has not been observed clinically, with MSC treatment of ongoing GVHD reported to have significant efficacy, while MSC prevention of GVHD not shown to be effective clinically.